1. Field of the Invention
The present invention relates to a method of forming an antifouling or antibacterial layer containing metal-modified apatite. The present invention also relates to an electronic device partially or entirely coated with such a layer.
2. Description of the Related Art
Portable electronic devices, in particular cellular phones, may always be carried by the users and frequently touched for operation, in comparison with much heavier household appliances such as washing machines or refrigerators. Eventually, particular parts of the portable devices will become dirty by the sebum of the user""s hands and further by the dust accumulated on the sebum. If the user is a smoker, the device may also be contaminated by the tar of cigarettes. Unfavorably, these accumulated dust or cigarette tar often gives rise to the proliferation of bacteria at the contaminated portion of the device.
In light of the above, recent portable electronic devices are often subjected to antibacterial treatment before they are put on the market. The desired antibacterial function can be provided by using a photocatalytic semiconductor in producing an outward coating of the device.
In a photocatalytic semiconductor, an electron can make a transition from the valence band to the conduction band when the semiconductor absorbs radiation having an energy corresponding to the band gap between the valence band and the conductor band. Due to the transition, a positively charged hole occurs in the valence band. When a pollutant comes into contact with the semiconductor, the electron present in the conduction band will move onto the pollutant, thereby reducing the pollutant. Then, the hole in the valence band strips the electron from the pollutant, to oxidize the pollutant.
When TiO2 is used as a photocatalytic substance, the transit electron in the conduction band reduces the oxygen in the air, thereby producing a super-oxydo-anion (O2xe2x88x92). On the other hand, the hole occurring in the valence band oxidizes water present on the surface of the TiO2, thereby producing a hydroxy radical (OH), which is a very strong oxidative molecule. Thus, while an organic compound is held in contact with the TiO2, it will be decomposed to water and carbon dioxide. Such an organics-degrading material as TiO2 is widely used for antibacterial or antifouling purposes.
Japanese patent application laid-open No. 11(1999)-195345, for example, discloses an antibacterial technique in which photocatalytic TiO2 is applied to the push buttons of an electronic device. Unfavorably, TiO2 has a weak adsorbing power with the contaminants. Thus, to put the antibacterial or antifouling function of the TiO2 to effective use, the contaminants need to be held in proper contact with the TiO2.
Japanese patent application laid-open No. 2000-327315 discloses a contact-improving technique between contaminants and TiO2. Specifically, the JP document teaches that TiO2 is combined, on the scale of atoms, with highly adsorptive calcium hydroxy apatite (CaHAP, or Ca10(PO4)6(OH)2), to provide metal-modified apatite. The metal-modified apatite has such a structure as obtained by partially replacing Ca contained in the crystalline calcium hydroxyapatite with Ti. The site of the introduced Ti locally provides a chemical structure resembling that of photocatalytic titanium dioxide. The combination of the xe2x80x9cquasi-titanium dioxidexe2x80x9d structure and the CaHAP is advantageous in that objectionable organics can be held in appropriate contact with the photocatalytic site by the adsorptive CaHAP.
Generally, a greater amount of metal-modified apatite needs to be used for more effective antibacterial or antifouling function. However, according to the method of JP No. 2000-327315, the metal-modified apatite is obtained as white powder. Thus, when the metal-modified apatite is used in an amount sufficient enough to take effect for antibacterial purposes, the resultant appearance of the electronic device may not be attractive due to the inevitable whiteness of the metal-modified apatite. Another problem of the conventional metal-modified apatite is that the particles of the metal-modified apatite powder tend to cohere and form a number of lumps in a solvent. Application of such a lumpy antibacterial material may lead to an unacceptably bad texture of the product.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a method of forming an antibacterial or antifouling layer which contains metal-modified apatite but still has a great transparency. Another object of the present invention is to provide a coating agent used for the method. The present invention also provides an electronic device having its part or parts covered by a metal-modified apatite containing layer.
According to a first aspect of the present invention, there is provided a method of forming a layer containing metal-modified apatite. In accordance with the method, an apatite-containing liquid is prepared, which contains both an inorganic coating agent and 0.01xcx9c5 wt % of metal-modified apatite powder. Then, the apatite-containing liquid is applied to an object.
The xe2x80x9cmetal-modified apatitexe2x80x9d mentioned above may be obtained by partially replacing a metallic element in the crystalline structure of a material apatite with a xe2x80x9cphotocatalytic metal element.xe2x80x9d A metal element is referred to as xe2x80x9cphotocatalyticxe2x80x9d when it can serve as the photocatalytic center in an oxide. Preferably, the inorganic coating agent may be highly transparent, having a transmission rate of no smaller than 90% for visible light.
The material apatite used to produce the metal-modified apatite of the present invention can be generally expressed by the empirical formula: Ax(BOy)zXs. In certain applications, the symbol A may be replaced by metallic elements such as Ca, Co, Ni, Cu, Al, La, Cr and Mg, the symbol B by P or S, and the symbol X by a hydroxyl group (xe2x80x94OH) or halogen (F, Cl, etc.). Examples of the material apatite may be metallic salts of hydroxyapatite, fluorapatite and chlorapatite, and others may be tricalcium orthophosphate, dibasic calcium phosphate, etc. According to the present invention, preferably the material apatite may be hydroxyapatite, in particular calcium hydroxyapatite (CaHAP, or Ca10(PO4)6(OH)2).
CaHAP is highly adsorptive especially with respect to organics such as proteins. Also, CaHAP strongly adsorbs mold and bacteria, thereby preventing them from proliferating.
Examples of photocatalytic metal elements are titanium (Ti), zinc (Zn), tungsten (W), manganese (Mn), tin (Sn), indium (In) and iron (Fe). When one of these elements is integrated into the crystalline structure of the material apatite, a xe2x80x9cphotocatalytic sub-structurexe2x80x9d is produced in the apatite crystal. More specifically, referring to the above-noted empirical formula [Ax(BOy)zXs], the photocatalytic sub-structure is provided by the combination of a photocatalytic metal element replacing the symbol A and the oxygen O in the formula.
In accordance with the method of the present invention, the resultant antibacterial layer contains a small amount of metal-modified apatite (0.01xcx9c5 wt %), which is advantageous to making the layer highly transparent. At the same time, it has been found that the layer of the present invention can function as a very effective antibacterial coating, in spite of the small amount of metal-modified apatite material.
Preferably, the preparation of the apatite-containing liquid may include a preliminary process in which a powder of metal-modified apatite is dispersed in an alcohol solvent. The preparation may also include a mixing process in which the alcohol solvent with the metal-modified apatite dispersed is added to an inorganic coating material.
Preferably, the metal-modified apatite dispersed in the alcohol solvent may be composed of a plurality of particles which are movable relative to each other, each of the particles having a diameter of no greater than 5 xcexcm.
Preferably, the particles having the diameter of no greater than 5 xcexcm may be obtained by ball milling or filtering of material metal-modified apatite.
Preferably, the metal-modified apatite may be obtained by partially replacing Ca of calcium hydroxyapatite by Ti. The Ti-CaHAP has both an excellent adsorbing power stemming from the CaHAP and an excellent photocatalytic power stemming from the titanium oxide.
Preferably, the inorganic coating agent may comprise heatless glass, which can form an inorganic glass coating at room temperature. The heatless glass may contain 70xcx9c80 wt % of an alcohol-solvable inorganic resin, 5xcx9c12 wt % of isopropyl alcohol, 3xcx9c4 wt % of methanol, and 2xcx9c3 wt % of dibutyl tin diacetate.
According to a second aspect of the present invention, there is provided a coating agent comprising an inorganic coating material and 0.01xcx9c5 wt % of metal-modified apatite powder.
According to a third aspect of the present invention, there is provided an electronic device comprising: a target portion of antibacterial treatment; and a layer covering the target portion. The layer contains an inorganic coating agent and 0.01xcx9c5 wt % of metal-modified apatite powder.
Preferably, the metal-modified apatite may be obtained by partially replacing Ca of calcium hydroxyapatite by Ti.
Preferably, the ratio of Ti to a total sum of Ca and Ti in the metal-modified apatite may be 3xcx9c11 mol %.
Preferably, the metal-modified apatite may comprise secondary particles each having a diameter of no greater than 5 xcexcm.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.